In a wireless communication system, data is transmitted from a transmitter to a receiver over a wireless channel, e.g. using microwave transmission.
To try to accommodate the increasing demands of data throughput, spatial multiplexing may be used by implementing a multiple-input multiple-output (MIMO) communication system. In MIMO, the transmitter has N transmit antennas and the receiver has M receive antennas. N transmit streams may be transmitted at the same time, each from a respective one of the N transmit antennas, and M receive streams may be received at the same time, each at a respective one of the M receive antennas. When the term MIMO is used herein, it also encompasses systems in which N=1 or M=1, i.e. single-input multiple-output (SIMO) and multiple-input single-output (MISO) systems. It also encompasses scenarios in which N=M where N and M are greater than one.
MIMO may be utilized in a microwave backhaul system. In a microwave backhaul system, the wireless channel is typically line-of-sight (LOS). As a result, all transmit streams may experience almost the same channel. Therefore, the received signals at the M antennas are typically highly correlated. To help enable spatial multiplexing in LOS MIMO, orthogonality of interfering received signals at the M antennas is desired. One way to try to achieve orthogonality for interfering signals is to focus on antenna geometric spacing. Ideally, to try to achieve orthogonality for interfering signals, the physical distance ht between adjacent ones of the N transmit antennas and the physical distance hr between adjacent ones of the M receive antennas should satisfy the relationship ht×hr=λD/2, which may be simplified to
      h    t    =            h      r        =                            λ          ⁢                                          ⁢                      D            /            2                              .      D is the physical distance between a transmit antenna and corresponding receive antenna in LOS, and λ is the wavelength of the wireless signals transmitted from the transmit antennas to the receive antennas.
For microwave backhaul systems in a traditional frequency band, e.g. long hops, meeting the optimum antenna spacing discussed above is typically impractical. For example, for a 10 km link having a carrier frequency of 13 GHz, the optimum antenna spacing is approximately 10 m, which is impractical to implement. Therefore, other techniques for reducing interference in a LOS MIMO system are desired.